An MRI apparatus is a modality necessary for medical care as of today, because it does not have a problem of exposure and can perform in-vivo examination in a less-invasive manner.
An MRI apparatus is an imaging apparatus which excites nuclear spin of an object placed in a static magnetic field with an RF (Radio Frequency) signal having the Larmor frequency and generates an image by reconstructing MR (Magnetic Resonance) signals emitted from the object due to the excitation.
Although high frequency signals (RF signals) are transmitted to an object in order to acquire the MR signals, the transmission of the high frequency signals warms up the object and increases the body temperature of the object.
Therefore, SAR (Specific Absorption Ratio) is defined as energy absorbed per unit mass of an object from the perspective of safety. Specifically, SAR (its unit is W/kg) is defined as energy of RF signal(s) absorbed in one kilogram of a living tissue, and safety reference values based on upper-limit values of SAR are defined in IEC (International Electrotechnical Commission) standard (IEC60601-2-33).
An RF signal is amplified by an amplifier so as to obtain a predetermined electric power and then transmitted to an object. This amplifier has nonlinearity and this nonlinearity becomes a factor of distortion of an outputted RF signal. As a result, it influences on setting accuracy of SAR.